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(11) | EP 1 482 317 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Earth resistance measurement instrument by neutral-to-earth loop and measurement procedure |
| (57) Earth resistance measurement equipment by neutral-to-earth loop, for installations
which have the generation and distribution earthed, permitting the measurement of
both the earth and neutral resistance, by means of injecting a high-peaked current
for a short period, incorporating additionally means of protection against overvoltages,
as well as a counter of transient overvoltages, and means of alarm, all of which is
incorporated in an equipment controlled by microcontroller and which has a screen
to display information on "abnormal" situations. |
OBJECT OF THE INVENTION
[0001] The present invention relates to an earth resistance measurement instrument by neutral-earth loop, permitting likewise measurement of the neutral impedance, as well as the procedure necessary for carrying out said measurement.
[0002] The present invention is characterised by the method for measuring the resistance of the earth conductor, which measurement is necessary to ensure that an earth fault provokes a homopolar current sufficient to activate the differential protection.
[0003] The equipment necessary to carry out the measurement of earth resistance and of neutral impedance can be installed in any part of the plant, on an electric panel for operation and control being preferable.
[0004] The system object of the invention is also characterised in that it includes the possibility of protection against overvoltages, as well as having a counter for these events, together with an alarm system, for the purpose of informing the user of the deficiency present in the facility.
BACKGROUND OF THE INVENTION
[0005] Until the present time, the protective systems existing in interior electrical facilities are based on protection against overloads, employing for this electromagnetic circuit breakers, which protect against overloads occurring in a continuous manner or against high currents produced by a short circuit. All facilities must also have means of protection against leakage currents, the object being to prevent any metallic part of a plant becoming electrically live. This protection is implemented by means of differential circuit breakers.
[0006] Nevertheless, on few occasions is equipment conceived for knowing the value of the neutral impedance and the resistance of the earth circuit, values which are important for the purpose of avoiding that an earth fault provokes a homopolar current sufficient to activate differential protective devices, as laid down in the standard IEE 713-12 (earth fault loop impedance).
[0007] Equipment is known like that described in the patent EP 881500, wherein a procedure is disclosed for the measurement of the loop impedance of a supply network with neutral conductor through differential and magnetothermal relay. The procedure is based on a first measurement of the line resistance, based on the variation of potential with respect to earth, by injecting a current between line and neutral. Thereafter a current is injected below that of the differential protection between neutral and earth. In both cases a current is injected below that of differential protection.
[0008] In the patent EP 706663 a method is disclosed for measuring the loop impedance of an alternating current circuit protected with a circuit breaker which comprises the stages of injecting a current of value below that of tripping the circuit breaker in order that it flows in the loop, this method permitting detection and isolation of a voltage signal that is developed in the measuring process.
[0009] Finally loop testers are known such as those disclosed in the patent GB 2292225 which are based on the difference in the current flowing though one of the lines and that returning through the neutral, the tester being connected to whatever socket, employing the principle of measuring the voltage in open circuit and with a resistance connected between line and earth, which will be of sufficient size so that the voltage drop is not sufficiently high to provoke the tripping of the protective means.
[0010] In all the aforementioned patents importance is given to measuring the loop formed by a line and the earth, but in no case is a measurement obtained of both the neutral and earth resistance, nor do they incorporate a means of protection against overvoltages and a counter of transient overvoltage events. And finally, they lack any means for indicating the state of the facility, since they are no more than procedures or equipment for knowing the value at a given instant of the earthing resistance.
[0011] Furthermore, the procedures disclosed in the aforementioned patents, do not take into account the possible influence of the variation in the voltage measurement due to the evolution of the injected current wave.
[0012] Thus, the objective of the present invention is to overcome the preceding drawbacks developing for this a system of earth resistance measurement which permits the neutral impedance to be known, which has means of protection against overvoltages, and means for indicating a deficiency in the supply mains.
[0013] Finally it is also pointed out that an object of the present invention is to overcome the drawbacks arising from the measurement procedure through not taking into account the evolution of the injected waveform in the variation of the measured voltage.
DESCRIPTION OF THE INVENTION
[0014] The system of measurement of earth resistance by earth-to-neutral loop consists basically in an equipment which will be installed preferably on the general operation and control panel, being possible to install on the secondary panels, which is intended for mains protected by residual over-current or differential circuit breakers, single phase or three phase, equipped with neutral earth and protective earth conductor.
[0015] Said equipment will be governed by a microcontroller which comes with a liquid crystal display which will inform of the neutral and earth resistance measurements, as well as of the state of the installation.
[0016] The measurement of the neutral and earth resistance is based on the injection of a high peak current, generated through the discharge of a capacitor limited by a resistance. Said current is injected across the neutral conductor (N) and the protective conductor (PE), for a reduced time, the object being that the effective current integrated over one half-cycle is less than that which trips the differential protection. The measurements of the neutral and earth resistance are calculated from the measurement of the variations in neutral (N) and earth (PE) with respect to the line (L).
[0017] With the object of evaluating the variation in potential, different timing diagrams are employed:
- Measuring the voltage prior to application of the impulse and during the impulse, the application of the current impulse taking place close to 90°, so that all the variation in the voltage is due to the impulse and not to the evolution of the half-wave.
- Measurement of the voltage in application and non-application half-cycles of current, also at 90°, injecting the impulses in alternate half cycles, so that all the variation between half-cycles is due to the current injection.
[0018] The variation of the neutral voltage on the terminals of the equipment will appear as a negative pulse, and that of earth as a positive pulse, since the current is injected from neutral to earth.
[0019] Moreover, as well as the measurements of earth and neutral resistance, the equipment carries out a periodic check of the installation making use of the values measured and the waveform, providing voltage values, of inversion of the line-to-earth, line-to-neutral connections, it being possible to indicate from the readings obtained the presence of the neutral in open circuit and of the earth.
[0020] The equipment also allows protection against overvoltages employing for this an overvoltage limiting device between line and neutral and between neutral and earth. The protection is of the type zinc-oxide varistor, gas arrester or a combination of the two.
[0021] Additionally it has a counter of transient overvoltages, which permits the number of times to be known that the earth voltage or the line voltages surpassed the limit for actuation of the protection, within a preestablished period.
[0022] Finally the equipment has an alarm for alerting an abnormality (very high neutral or earth resistance, voltage out of range, high number of overvoltages), activating in the event an output of the equipment which closes a contact that operates an alarm.
[0023] All the equipment comes with means for assembly on normalised DIN rails, capable of being plugged into earth sockets already existing.
DESCRIPTION OF THE DRAWINGS
[0024] To complete the description that is being made and with the object of assisting in a better understanding of the characteristics of the invention, attached to the present descriptive specification, as an integral part thereof, is a set of drawings wherein by way of illustration and not restrictively, the most significant features of the invention have been represented:
[0025] Figure 1 shows the schematic of the equipment for measuring earth resistance by neutral-to-earth loop.
PREFERRED EMBODIMENT OF THE INVENTION
[0026] In the light of the aforementioned figures, a preferred mode of embodiment of the proposed invention is described below.
[0027] In figure 1 the equipment object of the invention can be seen, which comes with a microcontroller (1) which serves to govern and control the assembly, to which is connected a liquid crystal display LCD (2) which informs of the transient overvoltages occurred in the earth resistance, on the state of the transient protection, on the state of the facility and of the network.
[0028] The equipment is connected through some terminal posts to the line conductor L, the neutral N, and that of protection PE, which proceed from a generating facility (12), having the characteristic that both the generating facility and the protective conductor are earthed either in the same place or separately, in this manner it is possible to carry out the measurement of the neutral and earth resistance through the earth / neutral loop.
[0029] A varistor (4) is likewise observed mounted between line and neutral, as well as a gas discharge tube (5), both serving for protection against overvoltages.
[0030] Thanks to the voltage dividers (6) and (7) the value of the voltage between line and neutral is known, as well as between line and earth when a current signal has been injected.
[0031] To know the number of overvoltages which occur in the installation, use is made of a filter (9) which permits said incidents to be detected.
[0032] Said current signal is injected on closing the contact (11) governed by the microcontroller (1). The power necessary for injecting the signal proceeds from a battery (10).
[0033] Finally there is a contactor (8) which, once closed, permits the supply of current to an alarm (3) serving to alert of the different situation of "abnormality'' (very high neutral or earth resistance, voltage out of range, high number of overvoltages, etc.).
[0034] It is not considered necessary to extend this description further for an expert in the matter to comprehend the scope of the invention and the benefits which derive therefrom.
1. Equipment for measuring earth resistance by means of neutral-to-earth loop for networks
with differential or residual overcurrent circuit breakers, single phase or three-phase,
with earthed neutral and with protective earth conductor, characterised in that the measurement of the neutral and earth resistance is carried out by the injection
of a current into the neutral-to-earth loop having integrated in the equipment means
of protection against overvoltages, a counter of transient overvoltages, means of
alarm for indication of "abnormal'' situations wherein the equipment on monitoring
the network voltage produces an alarm if the voltage is found to be outside the specified
ranges, and wherein the presentation is by means of display.
2. Equipment for measuring earth resistance by means of neutral-to-earth loop, according
to claim 1, characterised in that the presentation of "abnormal'' information is carried out by means of display.
3. Equipment for measuring earth resistance by means of neutral-to-earth loop, according
to claim 1, characterised in that it is mounted on the distribution panels on DIN rail.
4. Equipment for measuring earth resistance by means of neutral-to-earth loop, according
to claim 1, characterised in that the means of protection against overvoltages are zinc oxide varistors or gas discharge
tubes.
5. Procedure for the measurement of earth resistance by neutral-to-earth loop characterised in that it comprises the injection of a high peak current, generated through the discharge
of a capacitor limited by a resistance, injected across the neutral conductor (N)
and the protective conductor (PE), for a reduced time, the object being that the effective
current integrated over one half-cycle is less than that which trips the differential
protection, obtaining thereafter the measurements of the neutral and earth resistance
from the measurement of the variations in neutral (N) and earth (PE) potentials with
respect to the line (L).
6. Procedure for the measurement of earth resistance by neutral-to-earth loop, according
to claim 5, characterised in that with the object of evaluating the variation in potential use is made of the voltage
measurement of the voltage prior to application of the impulse and during the impulse,
the application of the current impulse taking place close to 90°, so that all the
variation in the voltage is due to the impulse and not to the evolution of the half-wave.
7. Procedure for the measurement of earth resistance by neutral-to-earth loop, according
to claim 5, characterised in that with the object of evaluating the variation in potential use is made of the measurement
of the voltage in application and non-application half-cycles of current, also at
90°, injecting the impulses in alternate half cycles, so that all the variation between
half-cycles is due to the current injection.